Cannabinoid receptors, CB1 and CB2, are part of the endocannabinoid system (ECS), which consists of cannabinoid receptors, endogenous endocannabinoids anandamide (AEA) and 2-arachindonoylglycerol (2-AG) and the hydrolytic enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL), a/b-hydrolase domain 6 (ABHD6) and a/b-hydrolase domain 12 (ABHD12) which are responsible for hydrolyzing AEA and 2-AG. FAAH is the principal enzyme for the in vivo degradation of AEA, while MGL, ABHD6 and ABHD12 together account for approximately 99% of 2-AG hydrolase activity. MGL colocalizes with CB1R in axon terminals and is responsible for approximately 85% of 2-AG hydrolysis (Savinainen, et. al., Acta Physiol. 2012, 204, 267-276). ABHD6 resides post-synaptically, often juxtaposed with CB1Rs, and its inhibition leads to activity-dependent accumulation of 2-AG. MGL and ABHD6 possess distinct sub-cellular locations in neurons which result in independent control of 2-AG accumulation and contribute to the fine-tuning of the magnitude and duration of the 2-AG signaling for the cannabinoid receptors. Specifically, when measuring 2-AG hydrolysis in neuron homogenates, ABHD6 and MAGL contribute about equally (Marrs et al., Nat. Neurosci. 13, 951-957, 2010).
The magnitude and duration of the in vivo CB1 and/or CB2 receptor modulation by 2-AG is relatively short, presumably due to its rapid inactivation process involving deactivating enzymes MGL (Karlson et. al., Biol. Chem. 1997, 272, 27218-27223) and ABHD6 (Marrs et al., Nat. Neurosci. 13, 951-957, 2010). MGL is a cytosolic enzyme that is also known for its ability to hydrolyze several bioactive fatty acid glyceryl esters not belonging to the endocannabinoid family, for example, 2-oleolglycerol and 2-palmitoyl glycerol. MGL plays dual roles in physiologic processes by regulating endocannabinoid tone as well as lipogenesis (Dinh, et. al., Proceedings of the National Academy of Science of the United States of America 2002, 99, 10819; Schlosburg et. al., Nature Neuroscience 2010, 13, 1113. ABHD6 is an integral membrane enzyme and a rate-limiting step of 2-AG signaling. Therefore, ABHD6 represents a useful target for modulation of cannabinoid receptors dependent disorders, alone or in conjunction with MGL.
Both cannabinoid receptors CB1 and CB2 belong to the GPCR family and have very different functions and distributions. The CB1 receptor is abundantly expressed in the central nervous system (CNS) and at lower levels in various peripheral tissues, including vascular and endothelial and smooth muscle cells, liver, skeletal muscle and adipose tissues (Herkenham, et. al., J. Neurosci. 1991, 11, 563-83; Egertova, et. al., J. Comp. Neurol. 2000, 422, 159-71; Bonz, et. al., J. Cardiovasc. Pharmacol. 2005, 41, 657-664; Mallat, et. al., Am. J. Physiol. Gastrointest. Liver Physiol. 2008, 294, 9-12; Pacher, et. al., Pharmacol. Rev. 2006, 58, 389-462; Howlett, et. al. Pharmacol. Rev. 2002, 54, 161-202; Mukhopadhyay, et. al., J. Am. Coll. Cardiol. 2007, 50, 528-536; Engeli, et. al., Diabetes 2005, 54, 2938-2843; Cota, et. al., J. Clin. Invest. 2003, 112, 423-431).
The CB2 receptor is mainly expressed in immune and hematopoietic cells and recently was also identified in the liver, and human coronary endothelium and smooth muscle cells, and at a lower level than CB1 in the brain. (Mukhopadhyay, et. al., J. Am. Coll. Cardiol. 2007, 50, 528-536; Mallat, et. al., Am. J. Physiol. Gastrointest. Liver Physiol. 2008, 294, 9-12; Rajesh, et. al., Br. J. Pharmacol. 2008, 153, 347-357; Van Sickle, et. al., Science, 2005, 310, 329-332; Gong, et. al., Brain Res. 2006, 1071, 10-23; Rajesh, et. al., Am. J. Physiol. Heart. Circ. Physiol. 2008, 293, 2210-22180.
The cannabinoid receptors CB1 and CB2 are involved in a variety of physiological or pathophysiological processes in humans and animals, e.g., processes related to the central nervous system, immune system, cardiovascular system, endocrine system, respiratory system, the gastrointestinal tract or to reproduction. Therefore, compounds which endogenously increase 2-AG levels are suitable for modulating these receptors and are useful in the prevention and/or treatment of cannabinoid receptor-related disorders. Conditions that may treated by modulation of the cannabinoid receptors include, for example, pain, neuropathic pain, neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, mental disorders such schizophrenia and depression, gastrointestinal motility disorders such as irritable bowel syndrome, coronary arteries disease, eating disorders such as anorexia nervosa and bulimia, also metabolic diseases such as obesity, diabetes, liver steatosis, metabolic syndrome, various inflammatory conditions and ocular diseases as well as other illness in which the cannabinoid system is implicated.
Naturally occurring cannabinoids as well as their synthetic analogues are employed for the modulation of the functional response of the cannabinoid receptors which translates to a physiological effect. The most widely used natural cannabinoid ∇9-tetrahydrocannabinol (∇9-THC), is the major bioactive constituent of Cannabis sativa (marihuana). Compounds can bind to the CB1 and/or CB2 in an individual or animal. There are well-established in vitro methods to assay the ability of a compound to bind to CB1 and/or CB2 receptors, as well as cell-based assays to study the functional response of the compound upon binding with the CB1 and/or the CB2 receptors (Abadji, et. al., J. Neur. 1999, 2 2032-20388; Dodd, et. al. Brain Res. 1981, 226, 107-18; Guo, et. al. J. Med. Chem. 1994, 37, 3867-3880; Morse, et. al. Life Sci. 1995, 56, 1957-1962, Lan et. al., J. Med. Chem. 1999, 42, 769-776).
The serine hydrolases ABHD6 and MGL hydrolyze the most abundant endocannabinoid (eCB) in the brain 2-arachidonoylglycerol (2-AG) and control its availability at cannabinoid receptors. In neurons, ABHD6 is located in postsynaptic dendrites at the site of 2-AG synthesis, where it fine-tunes the stimulated production of 2-AG and the resulting activation of presynaptic CB1 cannabinoid receptors (Marrs et al., Nat. Neurosci. 13, 951-957, 2010). MGL is localized in presynaptic axon terminals. Therefore, inhibitors of ABHD6 and/or ABHD6/MGL will independently control 2-AG accumulation and contribute to the fine-tuning of the magnitude and duration of the 2-AG signaling for the cannabinoid receptors and are useful in the prevention and/or treatment of cannabinoid-receptor related disorders. Recent studies suggested that ABHD6 inhibitors hold promise as therapeutics for obesity, nonalcoholic fatty liver disease, type II diabetes (Gwynneth et al., Cell Reports 2013, 5, 508-520) and insulin secretion regulation (Zhao et al., Molecular Metabolism 2015, 4, 940e950), traumatic brain injury (TBI) (Tchantchou et al., Journal Of Neurotrauma 2013, 30:565-579) and epilepsy (Naydenov et al., Neuron. 2014 16 83(2), 361-371).
The medicinal chemistry landscape of selective ABHD6 inhibitors and dual acting ABHD6/MGL inhibitors as pharmacological probes aimed at modulating 2-AG levels is limited.